1. Field of the Invention
This invention relates to electrographic processes for making color images. More particularly, this invention relates to electrographic processes and the elements used therein for the production of large size, full color images.
2. Description of Related Art
The use of electrographic processes in the manufacture of multicolor images is well known in the art. In such processes, an electrostatic latent image is produced directly by imagewise depositing charge onto an accepting dielectric surface. Typically, styli are used to create these charge patterns and are arranged in linear arrays across the width of the moving dielectric surface. Such processes and the required apparatus are disclosed, for example, in U.S. Pat. Nos. 4,007,489; 4,731,542; and 4,569,584. In U.S. Pat. No. 4,569,584, only one stylus array is used and the accepting surface web is traversed to-and-fro to make successive images, the toning stations being disposed on either side of the single charging station. In the other designated U.S. patents, the electrographic printer consists of three or more printing stations in sequence, each containing charging arrays and toning stations. In all of the disclosed processes, a multicolor toner image is assembled on an accepting surface of a support and fixed there for display.
A method for transferring a fixed image, such as an electrographically produced toner image, from an initial substrate to a final substrate is disclosed in U.S. Pat. No. 4,983,487. The disclosed method employs an adhesive-coated film to lift the image from its initial substrate and to secure it to the final substrate. The film remains in place after the transfer is completed and serves to encapsulate and protect the image. The initial substrate remains intact and may be reused.
A transfer process is disclosed in U.S. Pat. No. 5,102,768 for providing a non-electrostatically transferred toned image. In this process, an electrostatic latent image is conventionally formed on the surface of an element and that element is conventionally developed into a visible image by applying toner powder. The toned image is then thermally transferred from the surface of an element by contact to the face of a thermoplastic film that is strippably laminated to a paper or like backing. The film is then positioned against a receiver with the toner image therebetween, and the composite is subjected to two successive stages of compressive heating. It is disclosed that the process produces high resolution images from very small particle size toner powder on rough paper.
An offset transfer process of electrographically produced toner images is disclosed in U.S. Pat. No. 5,108,865. In the disclosed process, a liquid toned image is generated on the surface of an electrographic element. The image is adhered to the adhesive surface of a temporary receptor sheet which comprises a carrier layer, releasable release layer, and a transferable adhesive layer secured to the release layer. The temporary receptor sheet with the image adhered thereto is removed from the electrographic element, and then the image surface of the temporary receptor sheet is contacted with a final receptor surface. The adhesive layer secures the toner image, adhesive layer and release layer to the final receiving layer and the carrier layer is removed from the release layer to generate the final image wherein the release layer now is a top protective layer.
A toner developed electrostatic imaging process for outdoor signs is disclosed in European Pat. No. Publication No. 0437073 A2 (E.P. Application No. 90313976.4). This publication describes an electrographic imaging process (as contrasted to an electrophotographic process), in which electrostatic images are toned in sequence to form an intermediate image on a temporary dielectric receptor. The intermediate image is then transferred from the temporary dielectric receptor to a permanent receptor. In the disclosed process certain relative properties of the toner and the intermediate image, such as surface energy, Tg, work of adhesion, and complex dynamic viscosity, were identified as being important to the production of good final images.
Each of the electrographic processes disclosed in the patent publications discussed supra, employ a transfer of the toned image from an electrographic element to the final substrate using an intermediate transfer element. Although advances have been made in retaining the integrity of the toned image, such transfer steps remain prone to image degradation by abrasion or chemical interaction unless added laminating or coating steps are used. There continues to be a need for a simplified process to provide protected, distortion-free, full-color images, particularly, for use on large format posters, billboards and the like.